Clinical evidence has shown that implantable cardioverter defibrillators (ICDs) reduce mortality, which has led to a significant increase in their use. Patients implanted with an ICD, however, potentially face the problem of defibrillator shocks delivered unnecessarily for rhythms other than ventricular fibrillation or sustained ventricular tachycardia (VT). Unnecessary ICD shocks cause undue pain, psychological disturbance and can potentially induce more serious arrhythmias in some patients. Supraventricular tachycardia (SVT), including atrial fibrillation, atrial tachycardia and sinus tachycardia, are a common cause of unnecessary ICD shocks. The actual cause in a given case can often be ascertained by analysis of data recorded by the ICD. ICD shocks due to SVT being detected as VF or VT can account for some shocks delivered to ICD-implanted patients. Therefore, appropriately distinguishing SVT from VT could lead to a reduction in unnecessary ICD shocks being delivered, without decreasing ICD efficacy.